The sac fungi are separated into subgroups based on whether asci arise singly or are borne in one of several types of fruiting structures, or ascocarps, and on the method of discharge of the ascospores. Many ascomycetes are plant pathogens, some are animal pathogens, a few are edible mushrooms, and many live on dead organic matter (as saprobes). The largest and most commonly known ascomycetes include the morel (see cup fungus) and the truffle (q.v.). Other ascomycetes include such important plant pathogens as powdery mildew of grape (Uncinula necator), Dutch elm disease (Ophiostoma ulmi), and the chestnut blight (Endothia parasitica). Venturia inequalis, the cause of apple scab. Perhaps the most indispensable fungus of all is an ascomycete, the common yeast (Saccharomyces cerevisiae), whose varieties leaven the dough in bread making and ferment grain to produce beer or mash for distillation of alcoholic liquors; the strains of S. cerevisiae var. ellipsoideus ferment grape juice to wine.

Neurospora, a genus of about 12 widespread species, produces bakery mold, or red bread mold. It has been used extensively in genetic and biochemical investigations. Xylaria contains about 100 species of cosmopolitan fungi. X. polymorpha produces a club-shaped or fingerlike fruiting body (stroma) resembling burned wood and common on decaying wood or injured trees.

Cordyceps, a genus of about 100 species within the order Hypocreales, are commonly known as vegetable caterpillars, or caterpillar fungi. C. militaris parasitizes insects. It forms a small, 3–4-centimetre (about 1 ¹/₃-inch) mushroomlike fruiting structure with a bright orange head, or cap. A related genus, Claviceps, includes C. purpurea, the cause of ergot of rye and ergotism in humans and domestic animals. Earth tongue is the common name for the 22 Geoglossum species of the order Helotiales. They produce black to brown, club-shaped fruiting structures on soil or decaying wood.